A Guide to 12Volt Batteries

Batteries: their quirks & characteristics

After more than 150 years out there, the lead-acid battery is still the most popular and cost-effective rechargeable battery – by a long way. There are now sealed ones, starting batteries, deep-cycle ones, AGMs, Gel-batteries and heaps more variations, but they still all use the same basic chemistry of lead plates with sulphuric acid as electrolyte.

And then there are the others. The most talked-about are currently the Lithium-based batteries, and here too we have a number of variations which each have a niche application out there. They are much lighter, so in applications where weight is critical they have found a ready niche. So model planes now use brushless DC motors powered by Lithium batteries, and achieve things we could only dream of a few decades ago. In fact there are real-sized ones too – have a look at Solar Impulse to see what can be achieved when we put our minds to it.

Back on the ground we also have electric cars that use Lithium batteries, so we may be tempted to think that we should all be heading in that direction when it comes to our next battery for the ‘van. Well yes and no.

The yes is that they certainly are available – the no is that they are still expensive and potentially unstable. Some of us may remember the early laptop batteries catching on fire, and more recently some smart-phone batteries have reminded us that lithium batteries are still a relatively new technology when compared to the old lead-acid ones.

Comparing Lithium and Lead-Acid Batteries:

Currently the price-premium for Lithium batteries is enough to chase most of us away – they are still at least twice the cost of a lead-acid battery of the same Ah-capacity. There some smoke-and-mirrors around the sales of these batteries too. Some sellers of Lithium batteries claim that we should only use a lead-acid battery down to 70% of its capacity, so we can only use 30Ah out of a 100Ah lead-acid battery, otherwise the sky will fall in or something – nonsense!

A lead-acid deep-cycle battery is made to be discharged all the way down to zero. True, it prefers to be charged rather than discharged, but a deep-cycle battery is made for exactly what it says: to be cycled deeply, from 100% to zero and back again. So when comparing batteries we should do it on the basis of its capacity, measured in Amp-hours (Ah). Later we go into this a bit more under the heading of Different uses.

Lithium Battery Safety:

Let’s get back to the safety of Lithium batteries for a moment. Lithium is the lightest metal on earth, so it’s a good start if we want to keep things light, and with lead as the competitor it’s an easy race to win. However Lithium is also very unstable – in fact we have to keep pure Lithium under oil so it won’t spontaneously oxidise, and according to Wikipedia it is even used in rocket fuel. So until things get a bit more stable and there’s a bit more of a track record, personally I won’t be installing a Lithium battery in my ‘van thanks.

Charge and Discharge Currents:

That said, I’ve spoken to many people who are very happy with their Lithium battery banks, both in stationary and mobile systems – and I’m also happy to salute them as early adopters of this new technology. Aside from lightness, a Lithium battery also allows us to draw much higher currents – typically a lead-acid battery should not be discharged at more than about 15-20% of its capacity, so about 20Amps from a 100Ah battery. Depending on the exact type of Lithium battery, a 100Ah battery can typically deliver 100Amps without damage. The flipside is that it can also be charged more quickly than a lead-acid equivalent.

Battery Management System:

One last very important thing about Lithium batteries in a 12 Volt system – they must (must!) have a Battery Management System (BMS). Any reputable retailer will confirm this, and if they don’t I’d consider looking for another supplier. A typical 12 Volt Lithium battery will have four cells and the BMS ensures that each cell is balanced with the others. In other words the BMS makes sure the battery is never over-charged which risks explosion, nor is it over-discharged which risks the cell being permanently destroyed. So remember the three letters: B M S.

Lead-Acid Batteries:

So it’s back to the tried-and-trusted good old Lead-Acid batteries, and the first thing to realise is that there are two basic types of lead-acid batteries. They both use the same basic chemistry, but their internal construction is designed differently to suit one particular application.

Starting Batteries:

The battery we see when we open the bonnet of the car is a Starting Battery. It’s there to start the engine and that takes a heck of a lot of power but for a very short time. Now for the battery to be able to deliver hundreds of Amps to the starter motor, it needs to maximise the surface area of the lead plates, so manufacturers have found all sorts of tricksy ways to do this – for instance making them into a mesh or splitting each plate into a whole lot of thin fins.

For these starting batteries there are two important figures to look for: Cold Cranking Amps (CCA) and Reserve Capacity (RC). The first one is pretty obvious, and specifies the maximum current deliverable under cranking conditions. The second one (RC) is measured in minutes, and tells us how long the battery will last if it is supplying 25 Amps. The starting battery has a second job too – keeping things like headlights and wipers alive when the alternator is not supplying enough, for instance at idle or when the engine is off. So the RC figure gives us a measure of how well it will do its second job.

Deep-Cycle Batteries:

The other basic type of lead-acid battery is the Deep-Cycle battery. This comes in a number of different guises but they all have pretty much the same aim in mind – to last as long as possible when being discharged over a long period of time. So for deep-cycle batteries the manufacturers go for large, thick plates to maximise the capacity, which is measured in Amp-hours (Ah). Australian standards specify the Ah of a battery over 20 hours, so if we have a 120Ah battery and discharge it at 6 Amps then it should last for 20 hours (6Amps x 20hrs = 120Ah). Discharging it at a quicker rate, say over 10 hours, will mean we get less than the 120Ah because of additional losses as we hurry up the discharge process.

Expected Battery Life:

In deep-cycle batteries there are two main factors when looking at life expectancy. One is the so-called shelf-life which varies a bit from one make to another but is typically upwards of 5 years or so. This shelf-life assumes the battery has a pretty sheltered existence with no big demands or sudden shocks and is charged regularly and properly. The second factor in life expectancy is the Depth of Discharge (DoD) and this affects the number of charge-discharge cycles we can get from the battery over its lifetime. DoD measures what percentage of the battery’s capacity we have taken out – so if I have a 120Ah battery and use 30% of its capacity (36Ah) then I will have taken the battery to 30% DoD. I could also say that my battery now has 70% of its capacity left in it.

So, as we said before this second factor DoD affects the number of charge-discharge cycles we can expect from our deep-cycle battery. If we discharge the battery completely every time we use it, this will be a 100% DoD and we can then expect 250-300 cycles out of it However if we only take our battery halfway down every time, so to 50% DoD, then that number of cycles more than doubles to 600-700. And if we keep to just 30% DoD each time then that figure more than doubles again, so now we’re up at 1500-1700 cycles. Most battery suppliers have graphs or tables that show these figures, so you can confirm this for your particular battery.

So why do we care about all these cycles and shelf-life and stuff? Well let’s look at two different ways that we might be using our deep-cycle battery.

Different battery uses:

As a first example, let’s say we have a property out in the sticks somewhere and we’re using only solar to charge the 12Volt system’s batteries. If the batteries are completely discharged every day then we have 100% DoD and can expect to get less than a year out of our batteries – not great. But if we reduce this daily discharge to 30% DoD then we’re looking at more than 1500 daily cycles, or nearly 5 years – much better, and pretty close the battery’s shelf-life too. In reality the life of a battery will always depend on how the system is used, and there are regular tales of off-grid batteries lasting over 10 years.

As a second example, many of us and our 12Volt systems will be mobile – in motorhomes, campers, caravans, etc. What sort of life can I expect from my battery in this scenario? Well let’s take a situation where we manage to get away once a month – that’s probably optimistic for most of us, but there’s a reason I’ve gone that way. Now let’s also say that during the few days’ escape we completely flatten the battery, every time – makes sense because you want to use your battery to its full capacity. That means 100% DoD and only 200-300 cycles before my battery’s gone.

But wait, this only happens once a month, so I have 200-300 monthly getaways in that battery – and that’s about 20 years’ worth – so the battery’s shelf-life factor will kick in way before then. So the short version is that if I’m a once-a-month-camper I don’t have to worry about this depth-of-discharge thing at all. As I mentioned earlier in this article, lead-acid deep-cycle batteries are made to be discharged all the way down – no worries – just make sure it gets charged up again as soon as possible – batteries like being charged rather than discharged.

This diatribe on DoD also has a cross-over with the article on How much Battery? So for those looking at battery sizing and battery life, you’ll find more there.

Series and Parallel:

Putting two 12 Volt batteries in series doubles the voltage – this is where the one battery’s positive meets up with the other one’s negative, and the unconnected positive and negative are now the 24 Volt positive and negative. What does not change though is the battery capacity: two 12Volt 100Ah batteries in series will still give us 100Ah but at 24 Volts.

If we want to double the capacity but keep our voltage where it is, we can put our two 12V 100Ah batteries in parallel – this is where the two positives are joined and the same with the two negatives, so we have positive-to-positive and negative-to-negative. In this case we have 12 Volts at 200Ah, well in theory at least. I know many caravan manufacturers put batteries in parallel but that unfortunately doesn’t make it right.

Downside of batteries in parallel:

So why can’t I just put two 12V batteries in parallel? Well you can, but there’s a catch, and in most cases that I’ve seen it ends up shortening the battery life, sometimes considerably. The issue is this: each 12 Volt battery is made up of 6 cells, and part of the charging process involves trying to get these cells to all share the 12 Volts equally. This is done by holding the voltage just above 14 Volts until the cells have all found their own balance.

If we have two batteries in parallel, even if they are the same make and size and age, there will always be slight differences. These differences make it much harder for the two parallel-batteries to each balance its own set of six cells, and the result is that neither of the two batteries charges properly. It’s not the worst thing in the world to do, but it’s also not ideal. However it does help if the two batteries are separated every few months and charged up individually – that way they can each balance their own cells properly again.

The other argument against paralleling batteries is cost – one 12 Volt 200Ah battery costs less than buying two 12 Volt 100Ah batteries – simple. And don’t even think of putting an old and new battery in parallel – that will definitely end in tears – the old battery will just ruin the new one, and quickly too.

Battery State-of-Charge and Battery Voltage:

There is a rough relationship between the no-load voltage of a battery and its State-of-Charge (SoC), but I must stress that this is just approximate. The table below is however based on figures given by battery manufacturers, so it has a sound engineering base to it, it’s just that the exact voltages vary a bit depending on multiple factors.

Anyway, the no-load voltage is best measured when the battery has been disconnected from any loads for a few hours. This is often not practical, so again there is a compromise, but the battery voltage is always a lot better than guesswork. Just for fun I’ve put in the Depth-of-Discharge (DoD) figures too, so we can refer back to the discussions earlier in this article.

Battery Voltage (No Load)

State-of-Charge

Depth-of-Discharge

12.80 Volts

100% – full

0%

12.40 Volts

75%

25%

12.00 Volts

50% – half

50%

11.60 Volts

25%

75%

11.20 Volts

0% – empty

100%

NB: these approximate figures will vary with many things like temperature, age, brand, etc.

References:

Powersonic batteries: datasheet for 12V 80Ah battery

Trojan Batteries > tech support > battery maintenance

Wikipedia: Lead-acid batteries, Lithium-ion batteries and Lithium

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2 Comments

We have a 200Ahr LiFePO battery installed under the bed in our caravan with a Battery Management System and have used it for over a year and are very pleased with its performance. Safety wise we have less concerns than with Lead Acid battery chemistry. I have worked in the quality control laboratory of a lead acid battery factory.

Lithium batteries can be dangerous, but design of cases and management systems have improved so they are now used commercially and in the military. All modern batteries are dangerous to some degree, even if it is only due to them having high energy densities and the associated risks of rapid discharge.

Also do not forget LiFePO are inherently safer than any other commercially available Lithium battery, simply due to their safer chemistry and design. Lead acid batteries do have many problems and these are managed, by strong cases and design. Unfortunately they are heavy and contain corrosive acid and toxic lead, yet we use them in our cars, houses and caravans. So I think you need to be a bit more open to the idea of different battery chemistries.

We are currently seeing many new storage batteries coming on the market, once you get past the marketing hype many are still better than our commonly used batteries. So please do not dismiss a new technology because it has a reactive metal in it, many new technologies use toxic and highly reactive materials, but we manage the risks by good design and chemistry. A good example of managing a dangerous material is petrol, it is toxic, has an very high energy density can explode and readily ignites, yet we still use it and carry it in our cars and caravans.

Hi Terry,
Thanks for your comments, and especially for your kind words about the Blog.
LifePO batteries certainly are a leap forward over the lead-acid technology that’s been around so long, and given the advice-site nature of the 12Volt Blog we obviously have to err on the side of caution when it comes to new technologies.
This is why positive experiences like yours are so important in swinging the balance towards those new technologies – as I mentioned in the Blog, I’m always happy to salute you, and others I’ve spoken to, as early adopters – and your experience and knowledge of the battery industry no doubt helped in that too.
Thanks for sharing your insights with the readers of The 12Volt Blog.
Cheers
Alistair

PS: you may notice a subtle change I made to the post as a result of your comment.

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